Inflammation is the body's most fundamental and critical response to infection or injury. In response to inflammatory stimuli, the local endotheliu is activated by cytokines and chemokines to recruit circulating leukocytes to the areas of tissue damage. This involves a complex, stepwise sequence of adhesive molecular interactions that occur between leukocytes and the endothelial cells comprising the vasculature. Arguably the most important step of inflammation is the movement of leukocytes through the endothelial barrier itself, in a process known as transendothelial migration (TEM). PECAM and CD99 are two proteins that have proven to be critical for leukocyte TEM. While PECAM's function in this process has been studied in depth, very little is known about CD99 and the mechanisms behind its actions. In addition to being expressed on leukocytes and along endothelial borders, PECAM and CD99 also partially reside in an interconnected reticular network of membrane vesicles just underneath the endothelial plasma membrane, known as the lateral border recycling compartment (LBRC). During TEM, membrane from the LBRC (containing PECAM and CD99) is trafficked to sites of transmigrating leukocytes in a process known as targeted recycling (TR). TR is thought to supply the migrating leukocyte with unligated PECAM and CD99 to engage the cell and facilitate its movement out of the vasculature. Blocking PECAM function inhibits TR from being initiated and subsequently prevents TEM. However, it is not known if CD99 plays a role in this complex process. By defining how CD99 operates on a molecular level, we will gain great insight into not only how TEM is regulated but also how to possibly control this step of inflammation. While it has been shown that CD99 functions downstream of PECAM during TEM in vitro, the reports thus far regarding its level of function and significance in TEM in vivo have been largely conflicting. However, the conclusions from these studies have all been based on the interpretations of static ex vivo images and not by viewing the events in real time or in living tissue. In order to fully understand the function of CD99, it is critical to study exactly how it functions spatio-temporally to regulate leukocyte TEM. With our state-of-the-art 4- dimensional imaging system, we possess a unique and powerful system for studying the events TEM in real time in vivo at unprecedented resolution. By utilizing this technology, we will gain a more thorough understanding of this important molecule and gain novel insight into not only the physiologic mechanism of inflammation, but be able to identify a number of novel therapeutic targets for various pathologic conditions such as atherosclerosis, autoimmune diseases, and cancer.